Adding a new Roku to your current Roku account should go smoothly. Depending on how old your old Roku is the procedure may have changed from what you did before. During the setup you'll be asked to enter an email address. Make sure you use the same email address that your other Roku used. If you're not sure what it is, you can see it displayed under Settings->System->About on the old Roku. With any luck you'll receive an email from Roku with a link you have to follow to complete the linking process.
If you have any trouble just post here and someone will be sure to help.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution." If you appreciate my answer, maybe give me a Kudo.